The present invention relates to telecommunications devices and, more specifically, to systems for coupling telecommunications devices to telephone instruments.
A modem is a telecommunications interface that is used between a computer, facsimile machine or similar device and a telephone line to enable the device to communicate with another like device over the telephone line. Conventional modems use a two-wire connection that is typically connected directly to a two-wire telephone line. However, in telephone systems using a PBX arrangement, where many lines are serviced by a central controller, it is difficult to achieve dedicated line service for modem communications because the telephone lines carry line selection information and power in addition to the audio frequency communication signals.
U.S. Pat. No. 4,907,267, issued to Gutzmer, entitled "MODEM INTERFACE DEVICE," discloses a system for interfacing a modem to a PBX telephone system. The interface system is connected between the modem and the handset jack of a telephone instrument. The interface system thus provides audio frequency signals directly to the microphone input of the telephone instrument via the handset jack and receives audio frequency signals directly from the speaker output of the telephone instrument via the handset jack. The handset that was unplugged from the telephone instrument in order to plug in the interface system may be plugged into the interface system via another jack, and a switch is provided to allow either the modem or handset to be selectively connected to the handset jack. However, it is often desirable to use not the handset that was provided with the telephone instrument by its manufacturer, but a headset that a user can wear to allow hands-free conversation.
The optimal microphone input signal level for a telephone instrument may vary among telephone instruments produced by different manufacturers and even among different types of telephone instruments produced by the same manufacturer. An interface system that connects directly to the telephone handset jack thus may not work optimally with all telephone instruments. If the input signal level is fixed, that level may be too low for some telephone instruments and too high for others. A signal level that is too high or too low may cause data transmission errors when a modem is connected to the telephone instrument via such an interface system. In addition, oral communication may be impeded when a headset is connected to a telephone instrument if the headset is not specifically designed for compatibility with that telephone instrument because a signal that is too high or too low may render speech unintelligible.
Practitioners in the art have used circuits comprising multiple-position switches to manually select a microphone input level. A user of such a device may need to empirically determine which, if any, of the available input levels produces the fewest transmission errors when the interface device is connected to a modem or the clearest sound quality when the interface device is connected to a handset or headset. Such experimentation is clearly inconvenient and time-consuming.
A system for automatically adjusting the microphone input level to an optimum value would be highly desirable. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.